This invention is an extension and improvement of our previously invented method and apparatus U.S. Pat. No. 5,951,477 (the '477 patent) for single or single repeatable absolute intracranial pressure (ICP) value measurement and diagnosing of brain pathologies based on such measurements. This document is incorporated by reference in the present application.
The '477 patent teaches an apparatus and method for deriving an indication of intracranial pressure in a non-invasive manner using an ultrasonic Doppler measuring technique that is applied to the eye artery. In one aspect, this is achieved by a chamber which can apply a slight pressure to the eye and an ultrasonic apparatus which can simultaneously measure the internal and external blood flows in the eye artery. Signals representative of these velocity measurements, VI and VE are then compared and their difference, ΔV, is used to control the pressure in the chamber. When the pressure in the chamber causes ΔV to approach a desired minimum value, that pressure becomes an indication of the intracranial pressure.
One disadvantage of the method and apparatus taught in the 477' patent is that it is impossible to continuously and non-invasively monitor the absolute ICP value. Continuous monitoring of absolute ICP value is one of the aims of the US and EU traumatic brain injury management guidelines.
Therefore, one objective of the present invention is the continuous non-invasive monitoring of absolute ICP value. To achieve this objective, we non-invasively determine an absolute intracranial pressure value Poi in i-th measurement cycle using the method and apparatus taught in the '477 patent. This non-invasive measurement of Poi is then used as a single autocalibration procedure for the non-invasive ICP monitor during i-th time interval of ICP monitoring, and becomes the initial value of the absolute ICP scale for the next continuous absolute ICP monitoring cycle, Po(i+1). After the time of continuous ICP monitoring during (i+1)-th time interval, the next single autocalibration procedure is performed and new value PO(i+2) is identified. That value is used as the initial value of the absolute ICP scale for the next continuous absolute ICP monitoring cycle. This process is repeated for the desired number of monitoring cycles.
When several Poi data points are collected, a conversion factor Ω can be determined as a function of pulsatility indexes for a wider interval of absolute ICP values. Stability of the conversion factor dictates whether the time interval of continuous ICP monitoring should be decreased, or increased. If the conversion factor Ω is stable, the continuous absolute ICP monitoring time interval can be increased. If the conversion factor and the pathophysiological conditions of the patient are changing, the continuous absolute ICP monitoring time interval must be decreased.
Advantages of the present invention are that the absolute ICP monitoring is continuous and the external pressure Pe is used only for autocalibration of the system “individual patient—non-invasive ICP meter”. In the '477 patent, it was necessary to apply external pressure Pe to the eye for the entire sampling period of discrete absolute ICP monitoring. Thus, the added value of the invention is the possibility to obtain important information about absolute ICP value non-invasively and continuously between two single absolute ICP measurements which are used for the system's autocalibration.